1. Field of the Invention
This invention relates generally to induction furnaces and, more particularly, to a bushing cap for closing a gap in a bushing used to enclose an induction coil in a channel induction furnace.
2. Discussion of the Related Art
The principle of operation of a conventional channel induction furnace is similar to that of a transformer. An inductor coil is located around a laminated iron core which forms a core and coil assembly. This coil can be considered a primary winding. The material to be heated, typically an electrical conducting material such as metal is in the form of a metal loop surrounding the core and coil assembly and can be considered a single turn secondary winding. When an alternating voltage is applied to the coil, flux is induced into the laminated iron core. This flux induces a voltage, and a result thereof, a current exists in the metal loop. This current causes the metal loop to heat, melt and remain molten. To retain the molten metal in the loop, a refractory lining is provided about the loop. To keep the refractory from coming in contact with the core and coil assembly, a bushing is located about the core and coil assembly.
In order to achieve maximum efficiency, the molten metal loop must be closely positioned around the bushing. This close positioning requires a channel induction furnace design to have a minimal refractory thickness between the molten metal loop and the bushing. As such, it is well known in the art that a steep temperature gradient must be established in the refractory layer to achieve maximum refractory life. This is typically achieved by surrounding the core and coil assembly with a metallic liquid cooled bushing.
However, to prevent the liquid cooled bushing from also acting as a short circuited secondary winding, the bushing must have at least one gap placed along the entire length of the bushing. This is typically achieved by using an electrically non-conductive insulator in the bushing gap. Unfortunately the problem with using the non-conductive material is that it is also thermally less conductive than the metallic bushing. Therefore, the thermal gradient in the refractory layer is unfavorably altered about the bushing gap.
During normal operation of the furnace, molten metal penetrate the refractory layer around the molten metal loop. The penetrating metal forms a network or fin of molten metal in and about the refractory grains of the refractory layer. Over time, the molten metal network or fin progresses deeper into the refractory lining, thereby decreasing the thickness of the already thin refractory layer between the bushing and molten metal loop.
The rate and depth of the molten metal penetration is dependent on the thermal gradient in the refractory layer. Therefore, since the thermal gradient along the bushing gap is unfavorably altered by the material used to insulate the joint, the molten metal network or fin penetrates the refractory along the gap at a faster rate to greater depths than elsewhere around the bushing.
The molten metal network or fin may progress until it reaches the liquid cooled bushing or bushing ga insulator. If the molten metal reaches the liquid cooled bushing, it typically freezes on contact or slightly away from the bushing. However, because of the altered thermal gradient along the bushing gap and the increased rate and depth of molten metal penetration in this area of the refractory, most molten metal networks or fins will usually reach the insulating material within the gap instead of the liquid cooled bushing. When this happens, the molten metal does not freeze when it comes in contact with the bushing gap insulator. Rather, the molten metal thermally destroys the bushing insulator which then causes a molten metal run out at the bushing gap causing the furnace to be shut down.
What is needed then is a bushing cap to close the gap in an induction furnace bushing which will maintain substantially the same thermal gradient about the refractory layer as the liquid cooled bushing. Preferably, there should be no insulating material exposed to the penetrating molten metal network or fin. Accordingly, molten metal run outs should be prevented. Thus, an object of the prevent invention is to provide such a bushing cap.